This invention relates to a thermal limiter or fuse of the type which is particularly suitable for use in protecting electrical circuits employed in low power applications, and in confined areas. More specifically, this invention relates to an improved fuse which utilizes a material that fuses or melts at elevated temperatures in order to effect the opening of an electrical circuit.
It has long been customary to employ thermal fuses of the type which rely upon a spring-loaded device to open a circuit once the fuse has been activated or blown. Moreover, it is also conventional to employ a wax-like substance for retaining the spring-loaded device in its cocked or inoperative position, at least until such time that an increase in the ambient temperature causes the substance to melt and release the device, which in turn opens the circuit in which the fuse is connected.
Typically such prior thermal fuses employ some form of butt-type bridging contacts, which are parted by the spring-loaded device when the fuse is activated. All such fuses which employ butt-type contacts have the disadvantage that they are plagued by problems resulting from I.sup.2 R losses, which generate undesirable heat. Because of these I.sup.2 R losses the cutoff (circuit opening) temperatures may vary not only from fuse to fuse, but also from application to application.
Typical such prior art devices are disclosed in U.S. Pat. Nos. 3,727,164 and 4,145,654. Each employs a meltable pellet for retaining two separate contacts of a switching mechanism in electrical contact with each other until such time that a predetermined ambient temperature has been exceeded. This same principal, of course, may be applied to the normally-open circuit type of switches (see e.g., U.S. Pat. Nos. 4,084,147 and 4,160,968), because the meltable pellet can be used just as effectively to maintain two contacts spaced from one another against the resistance of a spring until the pellet melts. Still further variations of such fuses which employ movable or butt-type contacts can be found in U.S. Pat. Nos. 3,180,958; 3,291,216; 3,291,945 and 3,649,942.